Digital signals are transmitted and received in a recent optical communication system and a recent radio communication system. A receiving apparatus for a digital signal may compensate a waveform distortion using a digital filter on the received digital signal in many cases (for example, refer to PTL (patent literature) 1 and NPL (non-patent literature) 1).
In compensation of a waveform distortion, an FDE (frequency-domain equalization) circuit may be used for compensating a waveform distortion using a digital filter in a frequency domain after Fourier-transforming a received signal. PTL 2 describes an FDE circuit using a digital filter. FIG. 18 is a block diagram illustrating a configuration of an FDE circuit relevant to the present invention, described in PTL 2. An FDE circuit 990 includes a Fourier transform unit 991, a filter unit 992, and an inverse Fourier transform unit 993. The Fourier transform unit 991 Fourier-transforms an input digital signal Din(t), and outputs N signals with frequency components of 0 to (N−1) Δωs. The filter unit 992 performs waveform equalization processing in a frequency domain by multiplying these Fourier-transformed digital signals by filter factors. The output of the filter unit 992 is inverse-Fourier-transformed in the inverse Fourier transform unit 993. The inverse Fourier transform unit 993 outputs a digital signal with shaped waveform as an output signal Dout(t).
Furthermore, NPL 2 describes a procedure for controlling an FDE circuit by switching filter factors in semi-fixed manner using a look-up table (LUT) to compensate wavelength dispersion.